In situ denitrification and DNRA (dissimilatory nitrate reduction to ammonium) rates were measured in an earthen lined 5-cell integrated CW using 15N-enriched.
Geophysical Research Abstracts Vol. 17, EGU2015-15127-2, 2015 EGU General Assembly 2015 © Author(s) 2015. CC Attribution 3.0 License.
In situ denitrification and DNRA rates in soils and underlying groundwater of an integrated constructed wetland Mohammad Mofizur Rahman Jahangir (1,2), Owen Fenton (2), Eoin McAleer (2), Paul Carroll (3), Rory Harrington (4), Paul Johnston (1), Christoph Müller (5,6), and Karl Richards (2) (1) Department of Civil, Structural & Environmental Engineering, Trinity College Dublin, Ireland, (2) Teagasc Environment Research Centre, Johnstown Castle, Ireland, (3) Waterford County Council, Ireland, (4) Vesi Environmental Ltd., Co. Cork, Ireland, (5) School of Biology and Environmental Science, University College Dublin, Ireland;, (6) Department of Plant Ecology (IFZ), Justus-Liebig University Giessen, Germany;
Nitrogen (N) removal efficiency in constructed wetlands (CW) is low and again it does not in itself explain whether the removed N species are reactive or benign. Evaluation of environmental benefits of CW necessitates knowing N removal mechanisms and the fate of the removed N in such system. In situ denitrification and DNRA (dissimilatory nitrate reduction to ammonium) rates were measured in an earthen lined 5-cell integrated CW using 15N-enriched nitrate (NO3 –N) push-pull method. Measurements were conducted in 2 groundwater depths (shallow- soils in CW bed; and deep- 4 m below CW soils) in 2 contrasting cells (high vs. low nutrient loads) of the CW. Denitrification (N¬2O-N + N2-N) and DNRA were the major NO3 –N removal processes accounting together for 54-79% of the total biochemical removal of the applied NO3 –N. Of which 14-17 and 40-68% were removed by denitrification and DNRA, respectively. Both the processes significantly differed with CW cells indicating that N transformations depend on the rate of nutrient loads in different cells. They were significantly higher in shallow than deep groundwater. Environmental conditions were favourable for both the processes (i.e. low dissolved oxygen and low redox potential, high dissolved organic carbon, high total carbon and high dissolved organic N) but DNRA rate was favoured over denitrification by high ambient NH4+ concentrations, reduced sulphide and low pH (5.9 - 7.0). Low pH might have limited denitrification to some extent to an incomplete state, being evident by a high N2ON/(N2O-N+N2-N) ratio (0.35 ± 0.17, SE). Relatively higher N2O-N/(N2O-N+N2-N) ratio and higher DNRA rate over denitrification suggest that the end products of N transformations are reactive. This N2O can be consumed to N2 and/or emit to atmosphere directly and indirectly. The DNRA rate and accumulation of NH4+ indicated that CW is a net source of NH4+ in groundwater. Ammonium produced by DNRA can be fixed in soils and, when exchange sites are saturated, can flow to ground and surface waters. These results suggest that conventional input – output balance for N removal is misleading unless the removal mechanisms are fully understood and the fate of the removed N are known.
